Assembled disk drives are calibrated after assembly by a process known as read channel optimization. Read channel optimization is a self-test burn-in procedure, which is usually done at the point of manufacture for a disk drive. It is usually performed at a single temperature or temperature range, often between about 25° C. to 30° C. The inventors have found that the current RCO procedures lead to some problems for customers. Customers use disk drives at temperatures which may range from 0° C. to 55° C., and sometimes, beyond this temperature range. Before discussing the details of these problems, a general discussion of disk drive technology is useful.
The Tracks Per Inch (TPI) in disk drives is rapidly increasing, leading to smaller and smaller track positional tolerances. The track position tolerance, or the offset of the read-write head from a track, is monitored by a signal known as the head Positional Error Signal (PES). Reading a track successfully usually requires minimizing read-write head PES occurrences.
FIG. 1A illustrates a typical prior art high capacity disk drive 10 including actuator arm 30 with voice coil 32, actuator axis 40, actuator arms 50-58 and with head gimbal assembly 60 placed among the disks.
FIG. 1B illustrates a typical prior art, high capacity disk drive 10 with actuator 20 including actuator arm 30 with voice coil 32, actuator axis 40, actuator arms 50-56 and head gimbal assembly 60-66 with the disks removed.
FIG. 2A illustrates a suspended head gimbal assembly 60 containing the MR read-write head 200 of the prior art.
Since the 1980's, high capacity disk drives 10 have used voice coil actuators 20-66 to position their read-write heads over specific tracks. The heads are mounted on head gimbal assemblies 60-66, which float a small distance off the disk drive surface when in operation. The air bearing referred to above is the flotation process. The air bearing is formed by the rotating disk surface 12, as illustrated in FIGS. 1A-1B, and slider head gimbal assembly 60, as illustrated in FIGS. 1A-2A.
Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet actuator 20, interacting with a time varying electromagnetic field induced by voice coil 32, to provide a lever action via actuator axis 40. The lever action acts to move actuator arms 50-56 positioning head gimbal assemblies 60-66 over specific tracks with speed and accuracy. Actuators 30 are often considered to include voice coil 32, actuator axis 40, actuator arms 50-56 and head gimbal assemblies 60-66. An actuator 30 may have as few as one actuator arm 50. A single actuator arm 52 may connect with two head gimbal assemblies 62 and 64, each with at least one head slider.
Head gimbal assemblies 60-66 are typically made by rigidly attaching a slider 100 to a head suspension, including a flexure providing electrical interconnection between the read-write head in the slider and the disk controller circuitry. The head suspension is the visible mechanical infrastructure of 60-66 in FIGS. 1A to 2A. Today, head suspension assemblies are made using stainless steal in their suspension and beams. The head suspension is a steel foil placed on a steel frame, coated to prevent rust. It is then coated with photosensitive material. The suspension and flexures are photographically imprinted on the photosensitive material, which is then developed. The developed photo-imprinted material is then subjected to chemical treatment to remove unwanted material, creating the raw suspension and flexure.
Actuator arms 50-56 are typically made of extruded aluminum, which is cut and machined.
FIG. 2B illustrates the relationship between the principal axis 110 of an actuator arm 50, with respect to a radial vector 112 from the center of rotation of spindle hub 80 as found in the prior art.
The actuator arm assembly 50-60-100, pivots about actuator axis 40, changing the angular relationship between the radial vector 112 and the actuator principal axis 110. Typically, an actuator arm assembly 50-60-100 will rotate through various angular relationships. The farthest inside position is often referred to as the Inside Position denoted herein as ID. The position where radial vector 112 approximately makes a right angle with 110 is often referred to as the Middle Position, denoted herein as MD. The farthest out position where the read-write head 100 accesses disk surface 12 is often referred to as the Outside Position, denoted herein as OD.
Note that as illustrated in FIG. 2B, the X axis is preferably situated along the principal axis 110 of the actuator arm. The Y axis preferably intersects the X axis at essentially the actuator pivot 40. When the actuator positions the slider 100 so that the read-write head 200 is at MD, the radial vector 112 is essentially parallel the Y axis. Track 18 is shown near MD, but it should be noted that tracks exist from ID to OD, through out the disk surface 12.
FIG. 2C illustrates a simplified schematic of a disk drive controller 1000 of the prior art, used to control an assembled disk drive 10.
Disk drive controller 1000 controls an analog read-write interface 220 communicating resistivity found in the spin valve within read-write head 200.
Analog read-write interface 220 frequently includes a channel interface 222 communicating with pre-amplifier 224. Channel interface 222 receives commands, from embedded disk controller 100, setting at least the read_bias and write_bias.
Various disk drive analog read-write interfaces 220 may employ either a read current bias or a read voltage bias. By way of example, the resistance of the read head is determined by measuring the voltage drop (V_rd) across the read differential signal pair (r+ and r−) based upon the read bias current setting read_bias, using Ohm's Law. From hereon, the assumption will be that a read current bias is used. This is done to simplify the discussion, and is not meant to limit the scope of the claims.
Typically, channel interface 222 includes amplifying the difference in the read differential signals. The amplified difference is then adjusted to remove asymmetries in voltage swings. The output of the asymmetry adjustment circuit is then presented to a first filter. The first filter is controlled by a first cut-off frequency and also provides boost to further remove high frequency noise. Often, the filtered signal is then demultiplexed into a track servo signal and a data signal. Both the track servo signal and data signal are independently filtered and amplified. There are various points in which the signal enters the digital realm from analog, depending upon the specifics of the channel interface 222.
In FIG. 2C, channel interface 222 also provides a Position Error Signal PES to at least servo controller 240. The PES signal is used by servo controller 240 to control voice coil 32 to keep read-write head 200 close enough to a track 18 of FIG. 2B to support read-write head 200 communicatively accessing track 18.
It is now time to discuss read channel optimization. Read channel optimization is a self-test burn-in procedure, which is usually done at the point of manufacture for a disk drive. It is usually performed at a single temperature or temperature range, often between about 25° C. to 30° C.
Read channel optimization establishes optimal values for at least the following for each of a collection of track zones:                Write current and write current overshoot controls.        Read bias current.        Read channel gain, used to control amplification of the read channel analog signal.        Read channel filter cut-off frequency Fc.        Asymmetric balancing, so that the positive and negative swings of the read channel signal are balanced.        The filtered read channel signal, a servo track signal and a data channel signal.        FIR taps, typically 10 taps applied to at least one digitized stream, of the raw read channel, the demultiplexed data stream, and/or servo track stream. Typically, the FIR is applied to the digitized raw read channel stream before presenting the stream to a trellis decoder.        Trellis decoder seed values, initializing the track decoding of the filtered digitized stream based upon synchronization with a detected track header.        Servo track threshold values and filter cut-off frequencies.        
A track zone is a sequence of neighboring tracks. The collection of track zones encompass all the tracks of a disk surface with each track typically belonging to just one track zone. The disk drive accessing a track within a track zone is based upon the read channel optimized parameters, for that track zone.
Channel Statistical Measurements (CSM) are a standard system used in assembled disk drives to estimate channel quality, by measuring amplitude. Another quality measure is to determine the Bit Error Rate (BER). The track servo signal, various synchronization mark detection measures and error control coding estimates are often used to generate CSM. While BER is considered more accurate, CSM and BER have a strong correlation, making it possible to infer the BER closely from CSM measurements.
CSM is directly related to the channel characteristics, whereas BER is a higher level systems reliability/quality measure. Today, CSM has an advantage in built-in self test situations, in that channel interfaces can often automatically calculate CSM from relatively small test runs, whereas it takes much longer test runs for BER calculations to reach the same level of accuracy.
Consider an example situation where Fc is being tested. After tests of 1000 reads of the same track, one error is reported for Fc of 10 MHz, and two errors are reported for Fc of 12 MHz. The BER estimates are almost identical, because statistically, there is almost no difference in these results. However, for Fc=10 MHz, CSM is 423, and for Fc=12 MHz, CSM is 5023. These CSM figures indicate quickly that Fc=10 MHz is definitely preferred. To reach the same conclusion would take much longer for the BER approach.
By way of example, suppose the read channel optimization temperature was 25° C. Disk drives commonly include the ability to measure their operating temperature and use this capability to apply these heuristic formulas. When the temperature varies greatly from 25° C., many disk drive controllers 1000 will use a heuristic formula to alter one or more parameters based upon the operating temperature. While this is often better than doing nothing, the actual disk drive and its relationship with its environment are not taken into account.
One typical heuristic involves Write Current. Suppose the read channel optimization parameter for write current is 20 mA. A typical heuristic is to use a write current 5 mA higher for low temperatures and 5 mA lower for high temperatures. There are disk drives where this heuristic is not even close to the actual optimal values. Using a single heuristic does not take into account the actual disk drive.
Another example includes the cut-off frequency, Fc. Fc can vary from 10 MHz at 55° C. to 12 MHz at 25° C. to 18 MHz at 0° C. in some disk drives, and in others vary by less than 2 MHz across the entire operational temperature.
Note that disk drives are used in computers, which may experience temperatures well below zero or well above 55° C. Disk drive manufacturers cannot tell what environmental conditions a specific disk drive will encounter, at the point of manufacture. Additionally, a disk drive's operating environment may change dramatically over time.
The cost of test, and the uncertainty of the eventual environment a disk drive will be used in, preclude the utility of expensive extension of read channel optimization over several temperature ranges. Some disk drives are never subjected to freezing temperatures, while others never experience any temperature above room temperature in an air-conditioned office building. What is needed is a way to tune disk drive performance to match the operating environment, without raising the cost of testing the manufactured disk drive.
The inventors do not know of any manufacturer who has satisfied this need. The standard approach to this situation is for manufacturers to test disk drives under the worst case situations, which are considered to be extreme heat. If the disk drive performs adequately, it is shipped. While such an approach is to be commended for insuring the disk drives are operational when shipped, it has several limitations.
Disk drives are affected by temperature differently, particularly as the track densities and/or rotational rates increase. Different components respond to temperature in different ways. Testing at extreme heat does not address the effect of low temperature on the read-write head and air bearing upon which the read-write head flies over the rotating disk surface. The interface between air bearing, read-write head, and disk surface is affected differently from the permeability of the disk surface, for example.
It is a commonly observed fact that at least mechanical systems degrade over time. Initial read channel optimized values, which are the best values the manufacturers can provide at time of manufacture, have been found to no longer be optimal when the disk drive has aged several months or years. What is needed is a method updating the read channel optimization parameters to address the aging of a disk drive at the point of use.
To summarize, what is needed is a way to tune disk drive performance to match the operating environment, without raising the cost of testing the manufactured disk drive. What is also needed is a method updating the read channel optimization parameters to address the aging of a disk drive at the point of use.